Semiconductors are used in integrated circuits for electronic applications, including radios, televisions, cell phones, and personal computing devices, as examples. One type of semiconductor device is the semiconductor storage device, such as dynamic random access memory (DRAM) or a flash memory, both of which use charge to store information.
A more recent development in semiconductor memory devices involves spin electronics, which combines semiconductor technology and magnetic materials and devices. The spins of electrons, through their magnetic moments, rather than the charge of the electrons, is used to indicate the presence of a “1” or “0”. One such spin electronic device is a magnetoresistive random access memory (MRAM) device 100, sometimes referred to as a magnetic RAM, as shown in FIG. 1, which includes conductive lines (wordlines WL and bitlines BL) positioned in different directions, e.g., perpendicular to each other in different metal layers. The conductive lines sandwich magnetic stacks or magnetic tunnel junctions (MTJ) 102, which function as magnetic memory cells. FIG. 1 shows a perspective view of a portion of a prior art cross-point MRAM array 100. The MRAM device 100 includes a semiconductor wafer comprising a substrate (not shown). The substrate has a first insulating layer (also not shown) deposited thereon, and a plurality of first conductive lines or wordlines WL is formed within the first insulating layer, e.g., in a first wiring level.
In a cross-point magnetic memory device 100, each memory cell or magnetic tunnel junction (MTJ) 102 is disposed over and abuts one wordline WL. Each MTJ 102 includes three layers: ML1, TL, and ML2. The first magnetic layer ML1 is disposed over and abutting wordline WL. The first magnetic layer ML1 is often referred to as a hard magnetic layer or a fixed layer because its magnetic orientation is fixed. A tunnel layer or tunnel barrier layer TL comprising a thin dielectric layer is formed over the fixed layer ML1. A second magnetic layer ML2 is formed over the tunnel barrier layer TL. The second magnetic layer ML2 is often referred to as a soft magnetic layer or a free layer because its magnetic orientation can be switched along one of two directions. The first and second magnetic layers ML1 and ML2 may include one or more material layers.
Each MTJ 102 abuts a second conductive line or bitline BL over and abutting the second magnetic layer ML2, as also shown in FIG. 1, wherein bitline BL is positioned in a direction different from the direction of the wordline WL, e.g., the bitlines BL may be perpendicular to the wordlines WL. Array 100 of magnetic MTJs 102 includes a plurality of wordlines WL running parallel to each other in a first direction, a plurality of bitlines BL running parallel to each other in a second direction, wherein the second direction is different from the first direction, and a plurality of MTJs 102 disposed between each wordline WL and bitline BL. While bitlines BL are shown on top and the wordlines WL are shown on bottom of the array 100, alternatively, wordlines WL may be disposed on the top of the array and bitlines BL may be disposed on the bottom of the array.
The value of the resistance of a MTJ 102 depends on the way in which the magnetic moment of the soft magnetic layer ML2 is oriented in relation to the magnetic moment of the hard magnetic layer ML1. The resistance of MTJ cell 102 depends on the moment's relative alignment. For example, if the first and second magnetic layers ML 1 and ML2 are oriented in the same direction, as shown in FIG. 2B, the cell resistance Rc is low. If the first and second magnetic layers ML1 and ML2 are oriented in opposite directions, shown in FIG. 2C, the cell resistance Rc is high. These two states of the MTJ cell are used to store digital information (a logic “1” or “0”, high or low resistance, or vice versa).
The hard magnetic layer ML 1 is usually oriented once during manufacturing. The information of the cell 102 is stored in the soft magnetic layer ML2. As shown in FIG. 2A, the currents IWL and IBL through the wordline WL and bitline BL, respectively, provide the magnetic field that is necessary to store information in the soft magnetic layer ML2. The superimposed magnetic fields of the bitline BL and wordline WL currents have the ability to switch the magnetic moment of the soft magnetic layer ML2 and change the memory state of MTJ cell 102.
An advantageous feature of MRAM devices compared to traditional semiconductor memory devices such as dynamic random access memory (DRAM) devices is that MRAM devices are non-volatile. For example, a personal computer (PC) utilizing MRAM devices would not have a long “boot-up” time as with conventional PCs that utilize DRAM devices. Also, an MRAM device does not need to be powered up and has the capability of “remembering” the stored data (also referred to as a non-volatile memory). MRAM devices have the capability to provide the density of DRAM devices and the speed of static random access memory (SRAM) devices, in addition to non-volatility. Therefore, MRAM devices have the potential to replace flash memory, DRAM, and SRAM devices in electronic applications where memory devices are needed in the future.
With the down-scaling of integrated circuits, the formation of MRAM devices experiences problems. After the formation of MTJs 102, the respective wafer is exposed to the external environment. The materials of MTJs 102 are thus prone to oxidation, and hence the sidewall portions of MTJs 102 are oxidized. This adversely affects the performance of MTJs 102. Particularly, the R—H loop of MTJs 102 will become sloped. The problem is further worsened when the dimensions of MTJs 102 are reduced with the down-scaling of integrated circuits, since the oxidized portions become greater portions of MTJs 102.
A further problem is the effect of inherent stresses in MTJs 102 to their performance. After the formation of MTJs 102, process-related inherent stresses remain in MTJs 102. It is known that the inherent stresses affect the performance of MTJs 102. The stresses may not be in a desirable direction, or have a desirable magnitude. Theoretically, after the formation of MTJs 102, a post annealing could reduce the inherent stresses. However, this cannot guarantee the reduction of the inherent stresses. New MTJ structures and methods for forming the same are thus needed to solve the above-discussed problems.